The present invention relates to improvements in the cold fractionation of light gases. In particular it relates to a new and improved method for recovering ethene (ethylene) from cracking gas or the like in mixture with methane, ethane and other components requiring low temperature refrigeration. More particularly, it relates to the use of serially connected low temperature fractionating sections or dephlegmators and plural demethanizers to effect the separation.
Cryogenic technology has been employed on a large scale for recovering gaseous hydrocarbon components, such as C.sub.1 -C.sub.2 alkanes and alkenes from diverse sources, including natural gas, petroleum refining, coal and other fossil fuels. Separation of high purity ethene from other gaseous components of cracked hydrocarbon effluent streams has become a major source of chemical feedstocks for the plastics industry. Polymer grade ethene, usually containing less than 1 percent of other materials, can be obtained from numerous industrial process streams. Thermal cracking and hydrocracking of hydrocarbons are employed widely in the refining of petroleum and utilization of C.sub.2.sup.+ condensible wet gas from natural gas or the like. Low cost hydrocarbons are typically cracked at high temperature to yield a slate of valuable products, such as pyrolysis gasoline, lower olefins and LPG, along with byproduct methane and hydrogen. Conventional separation techniques performed at or near ambient temperature and pressure can recover many cracked effluent components by sequential liquefaction, distillation, sorption, etc. However, separating methane and hydrogen from the more valuable C.sub.2.sup.+ aliphatics, especially ethene and ethane, requires relatively expensive equipment and processing energy.
Plural stage rectification and cryogenic chilling trains have been disclosed in many publications, especially Perry's Chemical Engineering Handbook (5th Ed.), and other treatises on distillation techniques. Recent commercial applications have employed dephlegmatortype rectification units in chilling trains and as reflux condenser means in demethanization of gas mixtures. Typical rectification units are described in U.S. Pat. Nos. 2,582,068 (Roberts); 4,002,042, 4,270,940, 4,519,826, 4,732,598 (Rowles et al.); and 4,657,571 (Gazzi), incorporated herein by reference. Typical prior art demethanizer units have required a very large supply of ultra low temperature refrigerant and special materials of construction to provide adequate separation of C.sub.1 -C.sub.2 binary mixtures or more complex compositions. As reported by Kaiser et al. in Hydrocarbon Processing, Nov. 1988, pp 57-61, a better ethylene separation unit with improved efficiency can utilize plural demethanizer towers. Ethene recovery of at least 99 percent is desired, requiring essentially total condensation of the C.sub.2.sup.+ fraction in the chilling train to feed the distillation towers. It is known that the heavier C.sub.3.sup.+ components, such as propylene, can be removed in a front end deethanizer; however, this expedient can be less efficient than the preferred separation technique employed herein.